Study On The Photocatalysis Degradation Of Acephate By Nano TiO₂-UV Process

This work was supported by the Natural Science Foundation of Shanghai（10ZR1420700） and the Innovation Fund Project For Graduate Students of Shanghai（JWCXSL1022）. In order to improve the degradation rate of acephate, anorganophosphorus pesticide, based on the improvement of photocatalytic degradationreactor and the spectrophotometric determination method of organic phosphoruspesticide by phospho-molybdenum blue. The effects of factors, such as radiation modeand duration, TiO₂type and concentration, initial acephate concentration, temperature,initial pH value, H₂O₂, metal ions and gas ratio on the photodegradation efficiency havebeen systematically examined. Furthermore, optimization of the process has beenconducted through the Response Surface Methodology （RSM） and Reaction kineticshas been analyzed. The main research results were as following:The improvement of spectrophotometric determination of organic phosphoruspesticide by phospho-molybdenum blue was studied. The effects of wavelength,chromogenic reaction and coexisting ions were investigated. The sensitivity, accuracy,precision and stability of the improved method were compared with GB/T6913-2008.The results indicated that when the detection wavelength was chosen as890nm, adding2.00mL ammonium molybdate solution （26g/L）, and1.00mL ascorbic acid （100g/L,containing0.4g/L EDTA）, and incubated at45℃for10min, a good linear relationshipbetween absorbance （A） and phosphate mass （μg） was observed, which could beexpressed as Y=0.00453X-0.00229（R2=0.999）,and the apparent molar absorptivity is2.118×104L/（mol·cm）.The mass concentration of PO43-is0-160μg and conforms toBeer’s law.Mean recoveries of phosphate varied from95.20%to102.10%.The methodhas the merits of easy operation, and therefore is applicable to the requirements oforganophosphorus pesticide residues detection.Improvement of photocatalytic degradation reactor plays an important role in theoxidation of organic phosphorus pesticide. Utilized the degradation of acephate as anobject, we fistly discussed about illuminant species, reactor material, reactor structure,and the way to combine illuminator with the reactor, experiments have also beenconducted to find out the optimal conditions for the design of photocatalyticdegradation reactor. Besides, the new reactors have been build with quartz cryotrapping part, sodium calcium glass outer reactior and450W,365nm high-pressure mercurylamp. The evaluation suggested that photocatalysis of acephate pesticide for60min was96.55%.The photocatalytic degradation feasibility of acephate by TiO₂-UV process wasinvestigated. The effects of radiation mode and duration, TiO₂type and concentration,initial acephate concentration, temperature and initial pH value on the photodegradationefficiency have been examined. The photocatalytic effect was more efficient （99.9%）when the initial acephate concentration was20mg/L and in a suspension containing0.1g/L Degussa P25TiO₂, while keeping the reaction solution at25℃and continuouslyirradiated for80min. And it seems that an alkaline solution （pH=11） could improve thephotodegradation efficiency of acephate.In order to further improve the degradation efficency of acephate, based on theformer results of photocatalytic degradation of acephate, the effects of H₂O₂, metal ions,gas ratio, ethanol, ketone or the combination of H₂O₂and metal ions on thephotodegradation efficiency of acephate by TiO₂-UV process have been examined. Theaddition of gas with higher oxygen ratio, H₂O₂, Fe3+or Cu²⁺would be favorable for thedegradation, while the addition of ethanol or acetone would hinder the process.Accordingly, a degradation of more than98.03%of acephate would be achieved whenadding0.2mmol/L Cu²⁺,10mmol/L H₂O₂to the optimal operational conditions after20min irradiation, provided the oxygen ratio is80%.Based on the single factor results, the effects of TiO₂concentration, initial acephateconcentration, Fenton-like agent (H₂O_<sup>2+Cu²⁺) and reaction time on thephotodegradation efficiency of acephate have been optimized using response surfacemethod （RSM）, and a predictive model was developed and experimentally verified. Theresults indicated that the photodegradation efficiency of acephate by TiO₂-UV/Fenton-like process could be well predicted by a quadratic regression model. Althoughno synergistic effect existed between the molar ratio of H₂O₂:Cu²⁺and initial acephateconcentration, significant synergistic effects were observed among the other factors.Under the optimal conditions, i.e. TiO₂concentration0.1g/L, initial acephateconcentration23.09mg/L, molar ratio of H₂O₂:Cu²⁺26.34and reaction time25.42min,the predicted degradation efficency of acephate could be100.41%, which was in goodconsistency with the experimentally result （99.99%）. The results proved that RSM couldbe sucessfully employed to optimize the photocatalytic degradation process of acephate, and improve the photodegradation efficiency.The kinetics of the photocatalytic degradation of acephate could be well describedby Langmuir-Hinshwood model. The photocatalytic degradation rate of acephatefollows the first-order reaction kinetics under lower initial concentration conditions,while under higher initial concentration, the degradation would confirm to zero-orderreaction. At the same time, the kinetics of reaction are significantly influenced by thecatalyst amount, temperature, pH, air condition intensity, and the addition of H₂O₂andCu²⁺obviously, which would follow the first-order reaction kinetics.